-fno-strict-aliasing in CWARNFLAGS?

[Soeren Sandmann]

Dan Nicholson <dbn.lists at gmail.com> writes:

> Here's one link:
> 
> http://lkml.org/lkml/2003/2/26/158
> 
> > Traditionally, -fno-strict-aliasing was definitely necessary for the X
> > server and/or some drivers to work correctly.
> 
> I know in mesa it's been required. Here are two bugs fixed/worked
> around by -fno-strict-aliasing.
> 
> https://bugs.freedesktop.org/show_bug.cgi?id=6046
> http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=394311

I recently turned it on in pixman because completely reasonable code
like this:

    void
    pixman_contract (uint32_t *      dst,
                     const uint64_t *src,
                     int             width)
    {
        int i;
    
        /* Start at the beginning so that we can do the contraction in
         * place when src == dst
         */
        for (i = 0; i < width; i++)
        {
            const uint8_t a = src[i] >> 56,
                          r = src[i] >> 40,
                          g = src[i] >> 24,
                          b = src[i] >> 8;
    
            dst[i] = a << 24 | r << 16 | g << 8 | b;
        }
    }

is actually illegal under the C aliasing rules, and GCC can and will
break it unless you use -fno-strict-aliasing. I don't think any other
compiler makes use of type based aliasing, perhaps because they
rightly - consider the standard broken.


Soren